1. Field of the Invention
The present invention relates to a semiconductor device, a method for producing the semiconductor device, a sensor and an electro-optical device. More particularly, the present invention relates to a semiconductor device and a method for producing the semiconductor device, which utilize resistance control of an inorganic oxide film through energy light exposure and oxidation treatment, as well as a sensor and an electro-optical device employing the semiconductor device.
2. Description of the Related Art
In recent years, various types of flexible devices are receiving attention. The flexible devices have various uses, such as electronic paper and flexible displays. The structure of a flexible device basically includes a crystalline semiconductor or a metal film patterned on a flexible substrate, such as a resin substrate. Since the flexible substrates have a lower heat resistance than that of inorganic substrates, such as glass substrates, it is necessary to carry out the entire production process of the flexible device at a temperature not higher than the operating temperature limit of the substrate. The operating temperature limit of a resin substrate, for example, is usually 150-200° C., although depending on the material. Even a material having a relatively high heat resistance, such as polyimide, has the operating temperature limit of at most about 300° C. Therefore, it is difficult to directly form a semiconductor device which necessitates a thermal process carried out at a relatively high temperature, such as a thin film transistor using silicon, on a resin substrate having low heat resistance.
To address this problem, semiconductor devices employing oxide semiconductors, which can be produced by low-temperature film formation, are being actively developed.
In the oxide semiconductors, the carrier is generated due to oxygen deficiency. Therefore, depending on the solid state properties of the formed thin film, the off current increases if the semiconductor device is directly formed, and it is not possible to obtain sufficient properties of the semiconductor device. In a case where a high resistance film with reduced carrier generation is used, the off current can be reduced. However, for the oxide semiconductors, a resistance reduction (high-density carrier generation) technique, such as doping (impurity implantation) used for Si semiconductors, has not been established. Therefore, it is difficult to produce a semiconductor device by reducing the resistance only at the source region and the drain region to reduce the contact resistance between these regions and electrodes.
In such a situation, Japanese Unexamined Patent Publication No. 2007-142196 has disclosed, as a prior example, a method for controlling carrier density, in which a thin film containing zinc oxide and tin oxide, which is formed by a physical film formation technique, such as sputtering, is subjected to heat treatment under the presence of oxygen at a temperature that provides a temperature at the film surface being not lower than a temperature of the substrate during the film formation.
The method disclosed in the above patent document can reduce the off current. However, this method makes the entire inorganic oxide film have highly resistance, and even the regions connecting to the source electrode and the drain electrode, i.e., the source region and the drain region have an increased resistance value. Therefore, it is impossible to provide good contact between these electrodes and these regions.